1. Field of the Invention
The invention is in the field of manufacturing integrated optical components. More particularly, it relates to a method of manufacturing channel-type waveguides having Y-shaped branches in thin light-guiding layers having sharp vertices using mask materials which overlap one another and are applied in different stages of photolithographic processes.
2. Prior Art
Y-shaped branches, also referred to as Y-junctions, of channel-type waveguides are very much used in integrated optics. In symmetrical form, for example, they form part of 3-dB couplers, while in asymmetric form they in fact form the basis of mode-splitting or mode-filtering elements. They also occur, of course, in guides which intersect one another at an acute angle. A problem which is known in this connection is that, if the vertex between two branching conductors in such a Y-junction is not manufactured sharply enough, unacceptable losses in the optical signal and impairment of the function of the Y-junction occur. The sharpness with which the vertex of the Y-junction can be manufactured is, however, dependent on the quality of the photolithographic processes used in the manufacture and in the masks used in these processes. Reference [1] discloses a method of forming Y-junctions having sharp vertices on the basis of indium phosphide (InP) in which the two waveguide branches of the Y-junction are successively defined by selectively etching twice with wet-chemical etchants using masks which overlap one another. For this purpose, in an assembly of layers which comprises a substrate and pairs of layers, each pair having an etch layer and an etch-stop layer, a first part of a mask pattern, defining a waveguide pattern, is etched in the uppermost pair of layers in a photoresist with the aid of a first mask in a first etching step. The photoresist of the first mask is then removed. A second photoresist mask is then applied at least partially over the first part of said mask pattern. As a result of arranging for the masks to define two straight paths at the position of overlapping, a branching waveguide pattern having a sharp vertex in the underlying pair of layers is produced after a second etching step with the same wet-chemical etchants. A disadvantage of this known manufacturing procedure is the use of selective wet-chemical etchants. Such etchants etch the material to be etched either isotropically or along crystal planes. In the first case underetching is known to occur, with loss of accuracy and undesired profiling of the waveguides as drawbacks. In the second case, only those waveguides which follow the orientation of the material can be etched well. In crystalline materials, therefore, wet-chemical etchants are unsuitable for etching curved waveguides. Curved waveguides specifically require a perpendicular anisotropic etching, which can in fact be achieved with dry etching procedures such as that known as reactive ion etching. Since curved waveguides are often needed in integrated optical components as connecting guides at the branches of Y-junctions, such components cannot be manufactured well with this known procedure. In addition, this known procedure requires a complicated layer pattern to form, in particular, the pattern in the uppermost pair of layers, which serves as mask in the second etching step but at the same time etched away except at the position of the overlapping. At that point, a residue is left behind which cannot be removed further without attacking the remaining structure.